In a steel mill it is frequently necessary to pass a piece of strip stock through a processing stage, unwinding the strip from a coil of the processing unit and winding it into a coil again downstream of the processing unit. Thus a sheet-steel strip is unwound prior to being pickled and galvanized and is rewound after such a process. Similar such operations can be carried out on nonmetallic strips, such as plastic strips or textiles that must be treated or printed.
For continuous operation the trailing end of the strip of one coil is normally joined to the leading end of another coil, to avoid the necessity of rethreading the entire system for each coil to be treated. Similarly at the downstream end of the process the coil must normally be unloaded from the takeup mandrel when it reaches a certain size and another coil started. For both of these operations a so-called looper is employed to allow the strip to be stopped evn though at an intermediate location the strip is still moving.
A looper, therefore, normally has a fixed support carrying a plurality of guides and a movable support carrying another plurality of guides, the strip being looped zig-zag fashion back and forth between the guides of the one support and the guides of the other support. Shifting the movable support toward the fixed support allows the strip to move downstream of the looper while being fixed upstream of the looper and shifting of the movable support away from the fixed support allows the band to move upstream of the looper while being stationary downstream of it.
Accordingly it is necessary to be able to displace this movable loop support in either of two directions at speeds varying between a maximum speed in one direction and a maximum speed in the opposite direction. The displacement speed and direction of the movable support is normally determined by the strip velocities established by tension bridles that normally flank each such looping unit, one of which is provided upstream of the treatment location and another downstream.
The standard drive for such an arrangement is simply constituted by a rotary member, normally a winch drum or sprocket, and means such as a cable or chain connecting the periphery of this drum to the movable support so that the movable support is displaced at a rate dependent on the peripheral speed of the rotary member and in a direction depending on the rotation sense of the rotary member. The speed of displacement of the movable support toward and away from the fixed support establishes the tension in the strip, since it is essential for the looper to maintain even strip tension. This drive is normally constituted as a heavy-duty direct-current electric motor of the reversible type whose output is connected via a gear-type transmission to the drum or sprocket constituting the rotary member.
Such a system leads normally to substantial speed variation for the movable support and to substantial variations in tension in the strip. In particular at very low rotation speeds the direct-current motor normally operates pulse-wise, and these pulses are transmitted to the loops of the strip and, in fact, is reflected by them to a relatively large extent since several such loops are engaged over the movable support. Furthermore at low speeds the friction between gears in the transmission is considerable so that the system heats greatly. This irregularity at low speeds is particularly disadvantageous with relatively fragile strip stock, as for instance synthetic resins. It produces transverse waves in the strip and can even rip it apart altogether.